Documents may be "scanned" by an electronic image scanning device, in which a plurality of photosensitive devices are arranged to sense light imagewise reflected from a document to produce an electronic output indicative of sensed light intensity. In a simple arrangement, documents are placed on a transparent imaging support platen, and relative movement between the sensors and the document is provided, simultaneously with illumination of the document. As used herein, the direction of relative is referred to as the slow scan direction, while the direction of electronic acquisition of data is referred to as the fast scan direction. It is common practice to provide aa platen cover in association with the imaging platen to sandwich the document into fixed placement on the imaging platen, and to prevent intrusion of extraneous light. Generally, the platen cover portion that is placed adjacent the platen for the support of the document is colored to provide a uniform background.
It is often noted in the output of electronic image scanning devices, either at soft displays, such as video display terminals (VDT's), or in printed samples of the scanned information, that lines are not exactly parallel to the slow scan or fast scan directions. In FIG. 1, stepped or staircased lines A and B are compared with the non-stepped appearance of lines C and D, parallel to the fast scan direction. While this problem is not desirable, it is usually not unacceptable, when it is requied or expected. However, for lines that are desired to be parallel to the slow scan or fast scan directions, the stepped appearance is completely unacceptable. Unfortunately, it cannot be guaranteed that the document to be scanned provides the scanned image at the desired skew orientation. The image may not be placed squarely on the document, the edges of the document may not be squared, causing the image to be skewed on registration of an edge, or the operator may not precisely align the document at the correct registration position on the platen. All these situations may result in a skewed image and thus stepped lines.
It has been determined that the skew of the images in the typical staircase situations described above are not very great, and would require only minor adjustment skew correction. Unfortunately, it is not an adjustment easily accomplished by direct operator manipulation of a document. Additionally, subsequent to correcting the skew orientation of the document, closure of the platen cover may re-skew the document.
U.S.-A No. 4,682,242 to Sugita shows an arrangement wherein the sensor is mounted on a rotatable assembly to vary the skew between the document and the sensor. U.S.-A No. 4,013,362 to Beckman Jr. teaches an adjustable form guide providing registration edges that adjust to deskew the document. GB No. 22,469 (1905) and U.S.-A No. 2,510,363 to Anderson shows negative holders which may be rotated for the removal of skew during printing. U.S.-A No. 4,506,301 to Kingsley and US-A 4,318,135 to Allis at al. both provide arrangements which appear to allow rotation of the sensor in electronic image scanning devices for deskew operation.
It will be appreciated that while the stepped line appearance is characteristic of electronic image scanning devices, the problem of skewed lines degrading image presentation occurs in a wide variety of imaging techniques, including without limitation, electrophotography and other printing techniques, where it is desirable to provide lines parallel to the edge of a frame or sheet. Other situations where it is desirable to have to images closely associated, such as overlay or form printing, also suffer from the problem of skewed lines.